Malignant gliomas represent a difficult and challenging problem for oncologists. While roles for some chemokines in the biology of tumors are fast emerging, no published information is available on the involvement of the specific chemokine, fractalkine (FKN), and its receptor, CX3CR1, in intracranial tumors. We have identified FKN and CX3CR1 expression in the syngeneic murine GL261 glioma model. Since microglia represent the major CX3CR1-expressing cell in the CNS, these cytotoxic effector cells most likely underlie any role for FKN and CX3CR1 in the biology of intracranial tumors. The lack of highly selective CX3CR1 antagonists has hampered further understanding of FKN and CX3CR1 in these and other diseases. Our approach to this dilemma has centered on the development and characterization of modified virally encoded chemokine receptor antagonists based upon the CC chemokine encoded by human herpes virus 8, vMIP-ll. We have made a simple modification to the vMIP-ll sequence, inserting three amino acids (Asn-lle-Thr) between the first two conserved cysteine residues. The resultant peptide, vMIP-ll/NIT, displays enhanced affinity and selectivity for CX3CR1. We will take advantage of the similarities and differences between the CX3CR1 selective agonist, FKN, and the non-selective chemokine receptor antagonist, vMIP-ll, by generating and characterizing site-specific and chimeric FKN/vMIP-ll mutants. The results will shed insights into the properties of FKN that mediate affinity, selectivity, and signaling efficacy at CX3CR1. From this, high affinity selective CX3CR1 antagonists will be developed and their properties tested in in vivo models of intracranial tumor growth and host rejection. The aims are designed to: Evaluate intracranial GL261 tumor growth, necrotic index, and immune response after implantation of GL261 cells into mice deficient in CX3CR1. Determine structural features of FKN important for affinity, selectivity, and agonist activity at CX3CR1. Determine the antagonist properties of vMIP-ll/FKN chimeras and mutants in vivo using the intracranial GL261 glioma model.